The purpose of lubricating rolling bearings is to prevent metallic rolling elements from directly contacting metallic inner and outer races of the rolling bearings by forming a thin oil film on rolling surfaces and sliding surfaces thereof. Lubrication when effected to the rolling bearings brings about the following merits:                (1) Minimization of friction and frictional wear,        (2) Removal of heat evolved as a result of friction,        (3) Increase of the lifetime of the rolling bearings,        (4) Prevention of rusting, and        (5) Prevention of foreign matter from being trapped in the rolling bearings.        
In order to enhance those meritorious effects brought about by lubrication, it is necessary to adopt a lubricating method that is appropriate to the conditions under which the rolling bearings are used, to select a lubricant of a high quality and to design an appropriate sealing structure. Designing the sealing structure properly is required for removal of dusts contained in the lubricant, prevention of entrapment of foreign matter and avoidance of leakage of the lubricant.
In general, in the rolling bearing of a type used in rotatably supporting the machine tool spindle, the amount of lubricant oil used is extremely limited to a small value in order to minimize heat build-up resulting from stirring of the lubricant oil. FIG. 7 illustrates the friction loss and the bearing temperature relative to the amount of the lubricant used in such rolling bearing. As shown therein, the amount of the lubricant used is divided into five regions I to V In the lubricant amount region II, in which the temperature rise is lowest, an air/oil lubrication is employed.
In the air/oil lubrication, as shown in an example of the system in FIG. 8, the use is made of a reservoir 134 equipped with a level switch, a pump 135 and a solenoid valve 140 controlled by a timer 141 so that the lubricant oil can be metered accurately for each of the bearings 133 before it is supplied thereto at an optimum interval. After the lubricant oil has been mixed with air within an air supply tube 138 at a terminal end of an oil supply tube 136, the resultant air/oil mixture is jetted to a target portion of the rolling bearing 133 to be lubricated, by means of a nozzle 156 oriented towards such target portion. For this purpose, the air/oil lubricating system is largely employed as a lubricating method that is suitable to increase the rotational speed and lower the temperature rise of the machine tool spindle.
However, during the high speed rotation, the bearing temperature tends to increase, accompanied by lowering the capability of the lubricant oil to form an oil film. In addition, an air curtain formed by the air dragged to whirl around rotating elements of the rolling bearing 133 increases. Therefore, the higher the rotational speed, the more severe the lubricating condition, making it difficult for the lubricant oil, jetted from the nozzle 156, to get trapped inside the bearing 133. Because of this, the amount of the lubricant oil supplied in the air/oil lubricating system is so chosen as to secure a sufficient lubricant reliability during the maximum speed rotation.
With the air/oil lubrication, the amount of the lubricant oil to be supplied is determined in anticipation of the maximum speed rotation that requires the severe lubricating condition as discussed above.
On the other hand, during the low speed rotation, the bearing temperature is low and the capability of forming the air curtain which hampers the supply of the lubricant oil internally into the bearing is also low. For this reason, the lubricating condition is not so severe as that required during the high speed rotation, and the amount of the lubricant oil to be supplied is not required much. In other words, if the amount of the lubricant oil to be supplied to the rolling bearing 133 is determined so that an optimum lubrication condition can be obtained during the high speed rotation, such amount of the lubricant oil would become excessive during the low speed rotation. Since, with the air/oil lubrication, the amount of the lubricant oil falls within the lubricant amount region II in which change in temperature is extremely low, the bearing temperature will rise due to stirring drag if the amount of the lubricant oil to be supplied is excessive as described above.
The increase of the bearing temperature is also brought about by the following additional factors:
As an example of the air/oil lubricant supply, the inventor of the present invention has attempted to suggest a lubricating structure, in which not only is an outer peripheral surface of an inner race of a rolling bearing provided with an inclined surface area continued from a raceway surface of the inner race, but also a nozzle member is disposed along and spaced a predetermined distance from the inclined surface area so that the air/oil lubricant can be jetted from respective air/oil lubricant discharge ports of the nozzle member that are held in face-to-face relation with the inclined surface area. For the rolling bearing, an angular ball bearing or the like is used.
According to the suggested air/oil lubricating structure, the air/oil mixture, which is a lubricant oil mixed with a carrier air, is discharged from the air/oil lubricant discharge ports of the nozzle member and is then introduced into a gap between the inclined surface area of the inner race and the nozzle members. The air/oil mixture so introduced into the gap is subsequently guided into the bearing, being sucked by the effect of a negative pressure developed within the gap during the operation of the bearing, and further guided onto the raceway surface within the bearing and/or an inner peripheral surface of a ball retainer by the action of the surface tension of the lubricant oil, deposited on the inclined surface area of the inner race and a component of centrifugal force acting in a direction towards a large diameter portion of the inclined surface area.
As discussed above, since the air/oil mixture is supplied onto the inclined surface area of the bearing inner race and is not supplied directly onto the raceway surface along which rolling elements move, no wind sound resulting from revolution of the rolling elements is generated with the noise level consequently lowered. Also, since the lubricant oil is not supplied by an air spray, but the air/oil mixture supplied onto the inclined surface area of the bearing inner race is guided into the bearing by the effect of rotation of the bearing inner race, the air used merely serves to transport the lubricant oil to the inclined surface area of the bearing inner race and, therefore, the amount of the air used can be reduced. For this reason, the energy saving effect can also be expected as a result of reduction in the amount of the air used.
However, the above discussed suggestion has not led to disclosure of a specific angle of inclination appropriate for the inclined surface area of the inner race employed in the rolling bearing. In the air/oil lubricating structure such as suggested above, if the inclined surface area of the bearing inner race is designed with no regards paid to the angle of inclination, it has been found that the air/oil mixture deposited on the inclined surface area tends to be separated away from halfway the inclined surface area by the effect of the centrifugal force. Separation of the air/oil mixture away from halfway the inclined surface area of the bearing inner race leads to the incapability of the air/oil mixture efficiently reaching the rolling elements, resulting in lack of a sufficient amount of the lubricant oil. In such case, although it may be suspected that if the respective amounts of air and lubricant oil, both forming the air/oil mixture, are increased, the insufficient lubrication can be avoided, increase of the amount of the lubricant oil would bring about a risk of the increased stirring drag and the temperature rise. Where the bearing employing this air/oil lubricating structure is adopted to support, for example, a spindle device, increase of the bearing temperature brings about change in temperature of the spindle, thus affecting the precision of the spindle. Accordingly, it is necessary to avoid the unnecessary temperature rise by means of the efficient lubrication with a minimized amount of lubricant oil. Also, increase of the amount of air leads not only to increase of the energy consumption, but also a heavy load imposed on a compressor, which in turn increase the noises generated thereby.
Also, even where the above discussed air/oil lubricating structure is applied to such a cylindrical roller bearing 41 as shown in FIG. 24, if the angle of inclination of an inclined surface area 42b of an inner race 42 is not properly chosen, problems similar to those discussed above are unavoidable. In other words, a lubricant oil jetted onto the inclined surface area 42 and approaching a raceway surface 42a via the inclined surface area 42 will, after having reached a large diameter portion of the inclined surface area 42b as shown by the arrow A in FIG. 24, be separated from the inclined surface area 42b by the effect of the centrifugal force and will eventually be blocked off by an end face of a roller retainer 45 without flowing deep into an inner peripheral portion of the roller retainer 45. Because of this, the lubricant oil fails to reach within the interior of the bearing 41 and, hence, the bearing 41 will be lubricated insufficiently. Although even in this case the insufficient lubrication can be avoided if the respective mounts of air and lubricant oil are increased, increase of the amount of the lubricant oil appears to lead to increase of both the stirring drag and the temperature.